⭐ add(css_varriables): Add variables for bar height and color

[anselal]

I added 2 variables, and formatted the css file

[rstacruz]

Thanks - let me roll this in with the next version on #218.

I'd prefer the variable names to be prefixed (eg, --nprogress-color) so it plays well with other libraries. It'd also be great if there are non-variable fallbacks for the benefit of legacy browsers. I'm going to implement them over at #218.

Thanks for your contribution!